1. Field of the Invention
This invention relates to an evaluation mask, a focus measuring method and an aberration measuring method that are used in the field of semiconductor technologies.
2. Description of the Related Art
As a design rules for semiconductor devices to be manufactured have been miniaturized in recent years, a focus margin of a lithography process have been reduced. Therefore, the specifications of a surface flatness of a wafer and a field curvature of exposure apparatus have become more and more rigorous. By the same token, high precision techniques have been required for focus measuring methods and field plane curvature measuring methods using a resist pattern transferred on the wafer.
Conventionally, the methods described in U.S. Pat. No. 4,908,656 (to be referred to as USP1) and U.S. Pat. No. 5,300,786 (to be referred to as USP2) are used as focus measuring methods using a resist pattern along with other methods realized by applying either or both of them.
With the USP1 method, a rhombic pattern formed on a mask is transferred onto a wafer and the focus is measured by using the phenomenon that the transferred pattern shows a maximal size at the best focus. Conventionally this method is called SMP.
With the USP2 method, on the other hand, a so-called Levenson type alternating phase shift mask is used to expose to light an isolated stripe type shield pattern that is designed to show a phase difference other than 180° (optimally 90°) between the opposite ends thereof. Then, the image of the isolated stripe type shield pattern moves transversally when it is exposed to light at positive or negative defocus. The method utilizes this phenomenon for the purpose of measuring the focus.
The USP1 method can be applied not only to the focus measuring method of the isolated pattern but also a focus measuring method of a variety of periodic patterns. For example When the variety of periodic patterns includes two pairs of rhombic patterns which differ in size or two pairs of periodic patterns which intersect each other with an acute angle is formed by double exposures, the focus of the variety of periodic patterns can be measured by changing the period.
On the other hand, the USP2 method provides the advantage that the defocus with the proper (negative or positive) sign can be measured by a single exposure because the isolated stripe pattern moves substantially linearly relative to the best focal point.